From the Research
When prescribing a high estrogenic oral contraceptive (OC) and metronidazole to an 18-year-old female with simple partial epilepsy taking lamotrigine, the primary drug-drug interaction concern is the potential for decreased lamotrigine levels due to the estrogen component of the OC, which could lead to breakthrough seizures. Estrogen induces glucuronidation of lamotrigine, potentially decreasing its serum concentration by approximately 40-60% 1. This interaction is clinically significant and may require lamotrigine dose adjustments. Metronidazole does not significantly interact with lamotrigine, but it can cause a disulfiram-like reaction if the patient consumes alcohol while taking it. To manage this interaction, the lamotrigine dose may need to be increased when starting the OC, and then decreased when the OC is discontinued or during the pill-free week. Close monitoring of lamotrigine levels and seizure activity is essential during these medication changes. The patient should be counseled about potential breakthrough seizures when starting the OC and advised to report any increase in seizure frequency promptly. Additionally, she should be warned to avoid alcohol while taking metronidazole. The most relevant interaction to counsel the patient about is the addition of an OC can decrease the efficacy of lamotrigine. Some key points to consider include:
- The estrogen component of the OC can induce glucuronidation of lamotrigine, leading to decreased serum concentrations 2, 3, 4.
- The magnitude of this interaction can vary widely between individuals and may be influenced by other factors such as co-medication and genetic variability 4.
- Monitoring of lamotrigine levels and seizure activity is crucial to manage this interaction effectively 4. Given the potential for decreased lamotrigine efficacy, the correct answer is C: The addition of an OC can decrease the efficacy of lamotrigine.